In 2024, security researchers found an issue in the Linux kernel’s graphics subsystem, specifically in the Broadcom V3D (VideoCore IV 3D) DRM (Direct Rendering Manager) driver. This vulnerability is tracked as CVE-2025-21697. While it might seem minor at first—just a missing pointer reassignment—it can cause real problems for system stability and could be leveraged for attacks on affected systems.

In this article, we’ll walk through what happened, why it matters, and what you can do about it. We’ll look at the relevant code, see how it was fixed, and discuss potential exploitation.

What’s the Vulnerability?

Within the Linux kernel’s drivers/gpu/drm/v3d/ subsystem, jobs (GPU commands) are tracked by a pointer in the device structure. After a job is done, this pointer should be set to NULL, indicating no active job is in progress. If it isn’t, the driver might think an old job is still live. When you later unload the driver, this can raise a warning or worse, trigger use-after-free problems or let a malicious attacker exploit the dangling pointer.

In short:
If the pointer isn’t set to NULL, you could have a stale, dangling pointer—bad news if you care about stability or security.

Here’s a simplified view of what went wrong

// Inside the Broadcom V3D DRM driver
void v3d_job_complete(struct v3d_dev *v3d) {
    // Process job completion...
    
    // <-- BUG: The pointer is not set to NULL here!
    // v3d->job = NULL;  // This line missing!
}

If you unload the V3D kernel module at this point, the driver’s cleanup path might see the old job pointer. That can result in a warning like:

WARNING: v3d_job still active during driver unload!

Pressure on the driver (e.g., repeated loading/unloading, heavy graphics use) could trigger more severe consequences like use-after-free or invalid memory access.

The Fix

The kernel developers fixed CVE-2025-21697 by simply making sure the job pointer is set to NULL after the job is finished.

Patch snippet (see commit c471a6c):

void v3d_job_complete(struct v3d_dev *v3d) {
    // Finish processing job...
    
    v3d->job = NULL;  // CLEAR the pointer!
}

This lets the rest of the driver (and kernel) know there’s no longer a job running, so unloading the module or cleaning up works cleanly.

This kind of pointer bug can have real consequences

- Information leak or memory corruption: If an attacker is able to allocate something else at the old job’s memory location, and the driver later acts on the dangling pointer, the attacker could cause memory corruption or leak information.
- Stability/Denial of Service: In the worst case, the kernel could crash, or your graphics subsystem could become unstable—even just from normal use or via deliberate stress (e.g., repeated module loads/unloads).

Force the driver to unload (rmmod v3d).

4. Trigger code that dereferences the dangling pointer—potentially crashing the system or co-opting kernel memory.

Though there’s no public remote exploit for this bug, the risk grows if an attacker can run code on your machine.

How to Stay Safe

- Update your kernel! Fixed in kernel versions after mid-2024 (see commit link).
- Don’t unload the V3D module frequently on affected systems. If you’re stuck on an old kernel, avoid module hot-swapping.
- Monitor for kernel warnings. If you see messages about active jobs during driver unload, patch your system.

References

- Linux Kernel Git: Fix for cvd/v3d job pointer NULL assignment
- CVE-2025-21697 NVD entry *(Link may be pending if vulnerability is newly published)*
- drm/v3d Linux driver documentation

Conclusion

CVE-2025-21697 shows how even a minor oversight in pointer management can create potentially serious bugs in kernel code. Always NULL out pointers after use, especially those that track dynamically-allocated structures in critical paths. Upgrading to patched kernels is the best way to keep your system safe from these subtle but important issues.

Timeline

Published on: 02/12/2025 14:15:32 UTC
Last modified on: 02/14/2025 14:13:39 UTC